High quality CHDM modified poly(ethylene terephthalate) (PET) is difficult to manufacture. This is particularly true for those PET's modified with higher levels of CHDM, e.g. greater than 10 mol % CHDM. The choice of catalyst metals is essential to properly manufacture these copolymers with regard to particular properties, such as yellowness and clarity. Titanium catalysts tend to yield copolymers with a yellow coloration. Antimony and cobalt catalysts tend to be reduced to the metal, thus giving a grayish appearance to the copolymers and greatly reduces clarity. Good quality copolymers can only be prepared with the proper choice of catalysts, and the choices are not obvious due to interaction of a certain combination of catalysts.
It has been found that the choice of the proper catalyst system is dependent on the reaction process used. The choice of catalysts as a function of reactor residence time is not obvious because of the response of copolymer properties to the residence time and catalyst levels. Processes with relatively short residence time (shorter than about 2 hours) in the polycondensation reactor calls for an active catalyst system so that the molecular weights can build up quickly in the relatively short time available. Active catalysts such as titanium or gallium yield copolymers with the required rate of molecular weight increase. However, the rates of the side reactions are also fast, so that the resulting copolymer is unacceptably yellow. Stabilization with phosphorus yields good color copolymers but the reaction rate becomes unacceptably slow. Toning with blue toners such as cobalt reduces the clarity of the copolymers.
According to the present invention, a carefully chosen combination of catalysts provides these copolymers with low color and high clarity for reactions with relatively long residence times in the polycondensation reactor.
While all of the catalyst metals have been disclosed for the preparation of polyesters, we are unable to find any prior art that takes advantage of a combination of catalysts with the interactions as we have discovered. Furthermore, we have found no mention in the literature on the important effect of polycondensation time on the choice of catalyst systems and the properties of the copolymers.